Not entirely!!
There are two aspects to this.
1. Set up your own gsm gateway using gammu,kannel etc, and setup php handlers or kannel parser to get keywords and do relevant action like topup, callback etc.. In this setup, you may not necessarily have a premium SMS number, check with your local mob telco.
PLUS: You have full control over your application.Downtime limited by your SMS device.
MINUS:permutations and combinations of GSM modem/connectivity makes it complex. No chance of a premium shortcode number.
2. Get SMS from inbound providers to forward you keyword based SMS to your appropriate scripts.
PLUS: No setup hassles except script coding.
MINUS: Downtime dependent upon SMS provider.
Cheers!!
ashutosh
[email protected]